As equipment for effectively utilizing rainwater, there has been conventionally known one in which a water storage tank capable of storing rainwater therein is buried underground. Such an underground water storage tank is configured to collect rainwater fallen onto roofs or rooftops of houses or a surface of the ground through rain pipes or ditches, and to conduct and store the collected rainwater into the interior of the tank. Such equipment is provided with drainage means such as a pump capable of extracting water stored in the equipment, from the ground surface, and is configured to cause the water stored within the equipment to be extracted by the drainage means to utilize the water.
Conventionally, there has been known a water storage tank to be used in such equipment and installed underground in a relatively simple manner, which tank is configured with stacked water-bearing materials comprising sand, gravel, broken stones, or the like. This water storage tank is formed by: excavating a place where the underground water storage tank is to be formed; arranging a sheet onto the lowermost portion at the center of the excavated place, and stacking up water-bearing materials comprising sand, gravel, broken stones, or the like at the central portion of the sheet; thereafter, raising up the periphery of the sheet to thereby surround the periphery of the water-bearing materials; and placing the ends of the sheet onto upper surfaces of the water-bearing materials. This underground water storage tank is configured to store water in gaps between the water-bearing materials comprising sand, gravel, broken stones, or the like.
However, the above-mentioned underground water storage tank exhibits a problem that the amount of stored rainwater or the like is relatively small because of a relatively large amount of the water-bearing materials comprising sand, gravel, broken stones, or the like in the space enclosed by the sheet. To solve this problem, it has been proposed to form water-bearing materials by molded bodies of plastics formed by molds, respectively (see patent-related reference 1, for example). In case of the water-bearing materials comprising the molded bodies of plastics, usage thereof allows for decrease of an occupying ratio of the water-bearing materials in terms of a volume enclosed by a sheet to thereby increase an amount of stored rainwater.
Patent-related reference 1: JP-B-4-35580 (Claims)